This invention relates to containers, and more particularly to improved method for forming deep drawn containers from paperboard. While of broader applicability, the invention is directed to improvements in forming paperboard containers having liquid impermeable surface coatings.
In the molding of containers from paperboard blanks coated with liquid impermeable materials such as, for example, polymeric materials, efforts at preventing sticking of such coatings to heated elements of the mold structure have involved use of release agents, such as, for example, silicone. Materials of this type, however, add to the cost of the raw materials, and, in amounts required to be effective, are not compatible with food and drug laws. Also, coatings of polytetrafluoroethylene, available under the trademark "Teflon", have been applied to the surfaces of the mold structure to prevent sticking, but tend to break down and become ineffective after short periods of usage.
U.S. Pat. No. 4,026,458, believed material to the examination of this application, teaches deep drawing a polymer coated paperboard container of the hereinabove mentioned type from a coated paperboard blank. The blank is disposed between heated mandrel and mold elements, and these elements are moved toward one another to form and set the container. Application of heat ensures setting of the paperboard blank so that it will not revert to its flattened mode. To avoid sticking of the heated mandrel to the coating, the temperature of the mandrel is maintained at a lower, non-sticking temperature than the temperature of the mold. For example, the mandrel is maintained below about 195.degree. F. while the mold is maintained at between 200.degree. F. to 350.degree. F. It has been found, however, that this lower temperature of the mandrel does not always ensure adequate setting of the paperbord fibers to achieve dimensional stability in the finished container.
It is a general objective of this invention to provide improved method for forming a coated paperboard container that consistently achieves full setting of the fibers without damage to the coating.